Small electric pumps, for example as used for circulating water in fish tanks and for draining water from washing machines, are well known. Such pumps are usually driven by sub-fractional horsepower single phase synchronous motors which have a long life and a simple construction allowing the stator to be fully sealed from the pump chamber and from the rotor of the motor.
Being a single phase synchronous motor, the motor can start in either direction and thus, the impeller of the pump is designed to operate in both directions. However, the starting torque is relatively low and the motor has difficulty starting under a full load. To overcome this problem, it is known to allow a limited range of free rotation between the rotor and impeller, usually up to about 360 degrees. This allows the motor to initially oscillate until it gains sufficient initial momentum before driving the impeller and its load. This is achieved by using a mechanical coupling, commonly referred to as a lost motion clutch, in which two spigots or teeth are arranged to interact. However, as the coupling allows free movement of slightly less than 360 degrees there is considerable noise and strain as the spigots crash together.
U.S. Pat. No. 4,861,240, sought to overcome this problem by having the spigots rotate within a sealed chamber containing a viscous fluid, e.g., an oil or grease, to absorb the knocking noise. As the volume of viscous fluid is relatively large and the pressure on the viscous fluid is also relatively large, this placed a great strain on the seal and introduced a possible source of contamination should the seal of the chamber fail during use, allowing the viscous fluid to mix with the fluid being pumped.
Thus, there is a desire for an improved coupling between a rotor of a synchronous motor and a working part such as an impeller of a pump.